A wide range of applications exist for materials which can be manually molded in liquid form and quickly converted into flexible elastomeric articles. Particularly in prototype and artistic work, there is a common need to quickly fabricate complex elastomeric forms using versatile bench top molding techniques. Often, refinement of artistic or technical concepts requires iterative fabrication trials to produce a desirable result, and methods are needed which are both cost effective and easy to implement. Liquid bench top molding techniques provide the necessary versatility, allowing immediate fabrication of complex forms from simple molds which can be quickly carved, machined, or cast.
Bench top molding, as used herein, is used to describe techniques which are primarily manual in nature, and involve pouring, setting, and removal of a final casting from a mold. Such techniques are differentiated from more advanced processes in that they normally do not include the application of pressure, centrifugation, vacuum, etc. to facilitate the formation of a void-free casting. Although any number of sophisticated processes exist for molding a wide range of materials, most are poorly suited to economical and immediate casting of specialty models, prototypes, props, etc., on a small scale basis.
While conceptually straightforward, the technical requirements for bench top moldable materials are stringent. Specifically, as the material is cast, it must be sufficiently fluidic to allow gravity filling of the mold and escape entrapped voids. In addition, it must, through some mechanism, subsequently undergo transition into a final, mechanically resilient, article. These requirements limit the scope of suitable options.
Given the enormous number of thermoplastic polymer resins available for injection/blow molding, extrusion, etc., it would seem that thermoplastic polymers would be well suited to forming bench top molds. On consideration, however, thermoplastic polymers with sufficient molecular weight to yield good mechanical properties, have characteristically high resistance to flow in the melt. Although deformable under high pressure (as in injection molding), typical thermoplastic resins are far too viscous for gravity pouring even at high temperature. Although there are some formulations which can be melt cast/poured into molds, fluidity is typically limited and centrifugation and/or vacuum degassing would be needed to eliminate void entrapment. Since bench top casting requires fluidity sufficient to insure gravity mold filling and elimination of entrapped voids, viable thermoplastic alternatives are severely constrained.
While some waxes produce a highly fluidic melt which can be effectively bench-top molded, and such materials are sometimes used for prototype work, they do not possess the elasticity or mechanical integrity for important applications. The Knit-Collins Company of Cleveland, Ohio, for example, specializes in waxes for this type of application. Waxes, however, are notoriously brittle and soften at warm temperatures. Even for application in theatrical props, or most art forms, they can only be used with great care to avoid damage, fracture, and softening in warm environments.
Aside from waxes, very few materials exist which produce fluidic melts appropriate for bench-top casting. Although some exotic examples exist (such as low melt point metals like indium), these are, in general, rigid, expensive, and mechanically unacceptable for typical prototyping and artistic needs.
For all of these reasons, bench-top molding is most commonly performed using materials which are initially fluidic but experience chemical cross-linking in the mold. In general, several classes of such materials are now in common use. These include 1) epoxies, 2) acrylics, 3) polyurethanes, 4) silicones and 5) similar polymers which can be chemically crosslinked. Typically, such materials “cure” within the mold either due to a reaction between components mixed prior to curing, the application of heat to the material, or through a reaction with air and/or associated moisture.
A wide range of silicone and urethane compounds, specifically engineered and sold for bench-top casting, are commercially available. BJB Enterprises Inc., (Tustin, Calif.), for example, offers castable urethanes ranging form RC-8004FR (85 shore D hardness with 800 cps mixed viscosity) to LS-15 (15 shore A hardness with mixed viscosity of 275 cps). In addition, they offer castable silicone formulations ranging from TC-5005 (shore A hardness of 10) to TC-5050 (shore A hardness of 50).
While these materials are specifically engineered and sold for application in bench-top molding applications, they suffer a number of important drawbacks. Among these limitations, mechanical properties are of particular importance. The base polymers associated with these compounds have inherent characteristics which limit formulation of compositions which are both soft and mechanically durable (tough). As is well known to those skilled in the art, silicone materials become highly notch sensitive (splitty) as softness is increased and urethanes can become tacky and creep sensitive in low durometer forms. These difficulties make it problematic to formulate very soft compounds from these materials, as evidenced by the general lack of commercial materials with Shore A hardness under 10.
Beyond mechanical limitations, these materials also suffer from other drawbacks. Once cross-linked in a particular physical form, these materials cannot be reused or reprocessed since they have experienced fundamental chemical changes. In addition, these materials are typically difficult to remove once molded (due to the rigidity and sticking to mold surfaces), and are extremely expensive for many prototyping applications.
For all of these reasons, it is clear that a reusable composition which can be melted and gravity poured (or otherwise manually applied) to produce castings would have great utility and value.